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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Objective

The aim of this study was to test the obesity–type 2 diabetes mellitus (T2DM) link in the context of longitudinal changes in body weight during the progression to diabetes in mature adult nonhuman primates (NHP).

Design and Methods

A colony of 245 adult rhesus monkeys aged 8-41 years with 179 males were used to define overweight in males as a body weight: ≥13.5 kg or body fat (BF) ≥18% and obesity as ≥16.5 kg or BF ≥27%, and overweight in nonpregnant females was identified as a body weight >8.5 kg or BF >21% and obesity as ≥10.5 kg or BF ≥30%. A subgroup of 48 males (24 T2DM and 24 age-matched non-T2DM) males were studied before and following the onset of overt T2DM for the effects of changes in body weight and obesity in inducing this conversion to overt T2DM.

Results

Three years before overt T2DM, mean body weight was 18.4 ± 3.3 kg. The DM-destined group body weight was 3.2 ± 1.1 kg greater and had a longer duration and greater severity of obesity, with peak body weight reached at 3.2 ± 1.8 years before overt T2DM. At DM onset the two groups did not differ significantly in body weight or adiposity.

Conclusions

The natural progression from pre-DM to overt T2DM is caused neither by the amount of excess body weight at DM onset nor by the proximate increases in body weight/adiposity during the pre-DM period of impaired glucose tolerance. Obesity was, however, essential preceding all NHP cases that developed T2DM.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Increased adiposity is considered one of the major risk factors that contributes to or facilitates the development of the metabolic syndrome, type 2 diabetes mellitus (T2DM), cardiovascular disease, stroke, and cancer [1]. A meta-analysis of obesity and its comorbidities by Guh et al. analyzed 89 articles and identified 18 different overweight and obesity-associated comorbidities, among them, T2DM, endometrial and prostate cancer, coronary heart disease, and stroke in both males and females [6]. The Guh analyses, using BMI as the primary variable, showed that T2DM was strongly associated with overweight in males (relative risk [RR] = 2.40 (2.12-2.72)) and in females (RR = 3.92 (3.10-4.97)), and even more strongly with obesity in males (RR = 6.74 (5.55-8.19)) and in females (RR = 12.41 (9.03-17.06)) [6]. A large cohort study of 121,700 middle-aged (33-55 years) females showed that body weight, identified by increased BMI, is the single most important contributing factor for the development of T2DM [7]. The relationship between gain in weight or weight change and the development of diabetes was not addressed.

NHPs frequently develop spontaneous obesity, metabolic syndrome, and T2DM [8] with characteristics that are extraordinarily like those of humans [25]. The present colony of rhesus monkeys has been studied longitudinally for many years under constant dietary, activity, and environmental conditions. This consistency is a key consideration in discerning the characteristics of the DM progression and factors contributing to its overt expression. Therefore, these monkeys serve as an excellent animal model for longitudinal analysis of the pathophysiology and natural trajectories of obesity, metabolic syndrome, and T2DM.

The aims of this study were to further characterize the increased adiposity and obesity of middle-aged male and female macaques and the relationships between changes in adiposity and the development of T2DM under constant dietary and environmental conditions. Interactions between changes in body weight/adiposity and the development of T2DM were examined in a subgroup of males that developed diabetes while under longitudinal study and compared to age-matched nondiabetic adult rhesus monkeys that did not develop T2DM, which is very difficult to do in free living human subjects.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Primate colony characteristics and care

This rhesus monkey colony (Macaca mulatta) consisted of 245 individually housed monkeys (73% males for historical reasons). Monkeys were housed according to the Guide for the Care and Use of Laboratory Animals [14]. All primate care and experimental protocols were reviewed and approved by the Institutional Animal Care and Use Committee. The monkeys were provided with extensive environmental enrichment, such as treats, toys, music, videos, and human contact. The environmental conditions were consistent for all monkeys, and food intake was determined daily. All monkeys were fed ad libitum on a primate diet (Lab Diet 5038, Richmond, IN) (13.1% calories from fat, 18.2% calories from protein, and 68.7% calories from carbohydrates) and had access to water at all times.

The males ranged in age from 8.8 to 34.6 years and the females ranged from 8.8 to 41 years. Male body weights ranged from 8.8 to 26.5 kg, plus two outlier males weighing ∼31 kg. At a mean age of 16 years, the average male body weight was 14 kg. Among the males that had not lost weight because of aging or diabetes, the mean body weight was 17 kg. Among the females body weights ranged from 5.7 to 13.6 kg and at a mean age of 20 years, the mean body weight was 8.2 kg. Because of the major sexual dimorphism, males and females could not be combined in weight or body composition analyses. The colony has included 113 DM monkeys, with an average onset of DM at 18.8 ± 4.8 years (range of age of onset: 9.8-30.5 years), and the oldest insulin-requiring DM monkey was aged 41 years.

Dual-energy X-ray absorptiometry analysis

Imaging by dual-energy X-ray absorptiometry (DXA) scan (Lunar Prodigy Pro, Piscataway, NJ) was performed to allow accurate determination of total body fat (FM) and the total body fat free mass (FFM) with precise determination of changes in body composition across time. Two hundred nine DXA scans were performed on 97 rhesus monkeys (males n = 76, females n = 21; age range 8-30 years). The numbers available in the male group enabled the statistical analysis of the % BF and FFM compared to body weight across three subgroups of male monkeys: [1] normal monkeys (non-DM) (apparently normal with no evidence of metabolic disturbances and no diabetes); [2] prediabetic monkeys (pre-DM) (with all in this group meeting two or more of the following three criteria: weight ≥ 13.6 kg in males, Kgluc ≤ 2, and/or triglycerides ≥ 150 mg/day); and [3] diabetic (DM) monkeys (defined as having two or more fasting plasma glucose (FPG) levels ≥126 mg/dL).

Longitudinal prospective data analysis of body weight changes in relation to the progression to overt diabetes

On the basis of available longitudinal data obtained before overt diabetes, 24 male rhesus monkeys were selected from the 113 T2DM population, and these were compared to 24 age-matched males who never became diabetic and had been studied in the laboratory over the same age range (total of 48 monkeys).

Procedures

After an overnight 16-hour fast all blood samples were collected via peripheral intravenous catheters under anesthesia with ketamine (10 mg/kg body weight). Plasma glucose concentration was determined by the glucose oxidase method (glucose auto-analyzer, Beckman Instruments, Fullerton, CA and Analox Instruments, London, UK).

Statistics and analysis

All results are presented as means ± SD. Linear regression was used to determine the presence of a significant change in body weight, % BF, and/or FFM in non-DM, pre-DM, or DM rhesus monkeys. The Pearson correlation coefficient was used to test the relationships between body weight and age in relation to FFM and % BF in the 97 monkeys having body composition analyses. A linear mixed effect model and paired t-tests were used to determine body weight changes in the DM and non-DM age-matched groups at each 1-year period. Statistical analysis was performed using SPSS, 17.0 software. A P value of ≤0.05 was considered statistically significant.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Body weight and body composition analysis using DXA scans

On the basis of extensive longitudinal data in this colony of adult rhesus monkeys, we have identified the following criteria for classifying fully adult lean, overweight, and obese female rhesus: lean (body weight < 8.5 kg or % BF < 21), overweight (≥8.5 kg or % BF ≥ 21), and obese (≥ 10.5 kg or % BF ≥ 30). Male rhesus monkeys were considered lean if the body weight was <13.5 kg. This weight was associated with % BF < 18%. Overweight was defined as ≥13.5 kg, a weight range associated with a BF of about 18.1-26.9%. Obese male rhesus had body weights ≥16.6 kg. BF was usually >27% in monkeys above this weight (Table 1).

Table 1. Weight category of lean, overweight, and obese nonhuman primates (N = 113)
Weight categoryFemaleMale
Body weight (kg)Body fat (%)1Body weight (kg)Body fat (%)
  1. 1Percent fat represents the usual amount of fat at the given weight by sex, but individual variability means there may be some deviations from the estimated fat based on weight and vice versa.

Lean<8.5<21<13.5<18
Overweight≥8.5≥21≥13.5≥18
Obese≥10.5≥30≥16.5≥27

In adult male monkeys over the age of 8.8 years, body weight was strongly positively correlated with % BF (r = 0.80, P < 0.001) and with FFM (r = 0.75, P < 0.001) (Figure 1A and B). Among the males there was also a positive association between FFM and % BF (r = 0.31, P < 0.001) (Figure 1C). Among the small group of female monkeys over the age of 8.2 years (N = 21), body weight was positively correlated both with % BF (r = 0.85, P < 0.001) and with FFM (r = 0.73, P < 0.001).

image

Figure 1. Body weight, % body fat, and lean body mass in adult male rhesus monkeys over the age of 8 years, including normal animals (⋄). prediabetic (▪), and overtly diabetic monkeys (▴). Body weight was significantly and positively correlated with % BF (r = 0.80, P < 0.001) (A) and with FFM (r = 0.75, P < 0.001 (B). FFM and % BF were also associated (r = 0.31, P < 0.001) (C).

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When adult males were classified by metabolic status as nondiabetic (non-DM), pre-diabetic (pre-DM), or diabetic (DM), adult male monkey weights ranged widely within group as shown in Figure 2: Non-DM mean (±SD) body weight was 14.4 ± 3.1 kg, % BF 23.2 ± 11.4, and FFM 9.9 ± 1.6 kg. Pre-DM body weight was 17.2 ± 2.8 kg with an average % BF of 33.0 ± 6.7 and FFM of 11.3 ± 1.5 kg. DM body weight was 16.7 ± 2.8 kg, % BF 32.6 ± 6.7, and FFM 10.9 ± 1.4 kg. There were no differences between the body compositions of the pre-DM and DM groups; however, both groups were significantly heavier and fatter than the non-DM group.

image

Figure 2. In fully adult male monkeys over the age of 8 years, there was no association between age and weight (A), nor between age and % body fat, and between age and FFM (B). There was parabolic association between % BF and age (when the age range 3-30 years was considered; dashed line) and a negative association among the pre-DM and DM (22-30 years).

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Because the T2DM is a disease associated with aging, we examined the effect of age on weight, % BF, and FFM in adult male monkeys (Figure 2A-C). In males aged 8-30 years, there was no association between age and body weight or age and FFM (Figure 2A and C). Because of the tendency of younger adult monkeys (8-15 years) to increase in % BF and older monkeys (15-40 years) to decrease in % BF there is a parabolic relationship between age and % BF in the male NHP population (Figure 2B).

Among the female monkeys (data not shown), the DM monkeys were significantly lower in body weight (P < 0.05) and % BF (P < 0.01) compared to pre-DM monkeys. The relatively low BF of the overtly diabetic female animals reflected a much longer period of gradual weight loss after the development of diabetes and was associated with the severity of diabetes. One female with >10 years of insulin-requiring DM lived to be 40 years of age. Among the female monkeys (data not shown), the DM monkeys were significantly lower in body weight (P < 0.05) and % BF (P < 0.01) compared to pre-DM monkeys. The relatively low BF of the overtly diabetic female animals reflected a much longer period of gradual weight loss after the development of diabetes and was associated with the severity of diabetes.

The relationship between body weight and the onset of DM (FPG ≥ 126)

The mean body weight of the 24 longitudinally studied male DM monkeys, 5 years before meeting the diagnostic criteria for overt T2DM, was 17.3 ± 3.4 kg. Of these monkeys, 11 monkeys achieved their own peak body weights 3 years (18.6 ± 3.3 years) before reaching the diagnosis of overt T2DM. These peak body weights were reached at a wide range of ages (12.6-21.9 years). The individual weight trajectories of the eight male monkeys that were studied for at least 5 years before T2DM diagnosis are shown in Figure 3A. All monkeys lost weight before reaching overt T2DM and during the first year post-DM diagnosis, with a mean weight loss during this first DM year of −1.4 ± 1.1 kg, preceding initiation of insulin treatment (Figure 3A). The mean body weight at the end of the study was 14.3 ± 0.7 kg with an average −3.2 kg weight loss over the progression from pre-DM to overt DM and for 1 year beyond. No male rhesus monkey developed T2DM without having reached a body weight of at least 14.1 kg and a BF content of >22%. Therefore, to date no longitudinally studied rhesus (male or female) has been observed to develop T2DM unless the monkey had been at least overweight (or obese). No rhesus had been identified under ad libitum fed conditions to be lean at the time of onset T2DM rhesus; thus, overweight or obesity preceded all cases of T2DM among rhesus monkeys. Weight loss, however, clearly occurred during the late progression to diabetes, and in some cases was very significant before meeting the threshold for the diagnosis of T2DM.

image

Figure 3. Longitudinal analysis of the change in mean body weight of eight DM monkeys from 5 years before the diagnosis of DM (FPG ≥ 126 mg/dL) to 1 year after DM diagnosis (no insulin treatment) (A) compared to the mean change in body weight of eight age-matched non-DM monkeys (B). The mean body weight of 24 DM monkeys and 24 age-matched non-DM monkeys were represented as mean ± SD (C). There was a significant decrease in body weight during time −1 to 0 (P < 0.0001) and during time 0-1 (P = 0.0009) in DM group compared with the age-matched non-DM group. Only one failed so show weight loss during the 5-year period preceding overt T2DM.

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The 24 age-matched non-DM monkeys, studied over the same time period as those that developed T2DM while under longitudinal study, had a mean body weight of 15 ± 1.4 kg at the beginning of the study period and a mean of 16.5 ± 1.1 kg at the end of the period. The non-DM monkeys, age-matched to the DM monkeys, tended to gain less weight compared to the DM monkeys; however, the weight gain varied between monkeys, as shown in Figure 3B, for the eight non-DM monkeys age matched to the DM monkeys in Figure 3A. The average body weight trajectories for the DM and the non-DM monkeys beginning 5 years before DM and extending to 1 year after DM diagnosis are shown in Figure 3C. Statistical analysis showed no significant difference in weight change between the DM group and the age-matched non-DM group during the time period −5 to −1 years, P = 0.6361 because this was on average a steady-state period; some were gaining; some losing; and some were stable. From 1 year before DM onset until overt T2DM, there was a significant weight loss in the DM group compared with the age-matched non-DM group, P < 0.0001. In addition, the DM group showed a further significant decrease in body weight over the year following DM onset compared to the age-matched non-DM group, P = 0.0009 (Figure 3C).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

To develop new standards for overweight and obesity in rhesus NHPs, and to use these in examining the present colony (and for future prospective use in larger colonies without DXA analyses available), we have measured body weight, % BF, and FFM. Measurements were performed using DXA scans in both male (age 8-30 years) and female (age 8-30 years), non-DM, pre-DM, and DM rhesus monkeys. Before overt DM, in both males and females, the non-DM monkeys were significantly lower in body weight (P < 0.01), % BF (P < 0.05), and FFM (P < 0.05) compared to the pre-DM monkeys.

The average age of onset of T2DM in this colony was 18.8 ± 4.8 years (range 9.8-30.5 years). Nonhuman primates provide the unique possibility of examining directly and quantitatively the longitudinal changes in body weight and obesity combined with the timing of the onset of T2DM. Various studies have shown the greater accuracy and reliability of using the DXA scan in estimating BF and FFM when compared with other methods of estimating BF and FFM, such as the titrated water method (isotope dilution of tritium [3H2O]), total body potassium, bioelectrical impedance analysis, and skin fold measurements both in humans and rhesus monkeys [10, 15]. In this study, the relationship between body weight and body composition in males and females, and in non-DM, pre-DM, and DM male rhesus monkeys has been assessed using DXA scans (Figure 1). We have shown that in non-DM monkeys, % BF in both males and females and FFM only in males were significantly lower compared to pre-DM monkeys. Body weight was also significantly associated with both % BF and FFM in male and female monkeys. In addition, the body FFM significantly decreased with age when all monkeys were combined; however, % BF decreased with age principally in monkeys in the age range from 22 to 33 years (or older) (data not shown). Our current results confirm our previous report that showed significant associations between body weight and % BF in a small group of rhesus monkeys measured by the titrated water dilution method (N = 24, obese/nonobese monkeys) [10]. This study also agrees with Black et al., who studied relatively lean (6.7-12.3 kg) and young (5-8 years) male rhesus monkeys (N = 10) showing significant correlations between total body weight, fat mass, and lean mass in rhesus monkeys using DXA measurements [15].

In this study, body weight changes were analyzed over 1-5 years before the development of T2DM in rhesus male monkeys and compared to the body weight changes in age-matched male monkeys that never developed T2DM. Monkeys that developed T2DM had shown considerable weight loss over several years before the development of T2DM and well in advance of the increase in FPG to overt T2DM levels (>126 mg/dL). Monnier et al.'s study on early signs of diabetes using 24-hour glucose monitoring in 130 T2DM patients with variable DM severity concluded that the first sign of diabetes in humans with T2DM was postprandial glycemia [20, 21], while the fasting glucose was still below the diagnostic level of overt T2DM (6.3 mmol, 114 mg/dL) [20, 21]. They thus considered fasting hyperglycemia to be a late indicator (immediately preceding overt DM) of T2DM and prandial induced hyperglycemia leading by several years in the progression to T2DM. The weight loss observed in this study was most likely due to the intermittent postprandial hyperglycemia that developed ∼1-3 years before the appearance of fasting hyperglycemia. In humans and monkeys, such prediagnosis postprandial hyperglycemia has been reported frequently and also associated with increased glycosuria.

At the beginning of this study, monkeys that developed T2DM were significantly overweight (3.2 years before overt T2DM) compared to age-matched monkeys that never developed T2DM. Both modest (<10 kg) and substantial (>10 kg) weight gain by 8.8 years of age was associated with increased risk of the development of overt T2DM in subjects with normal glucose levels (<6.1 mmol, ≤110 mg/dL) [22]. Neither moderate nor substantial weight gain increased the risk of T2DM in human or monkey subjects with high glucose (>6.1 mmol, >110 mg/dL) [22] probably because, as shown in this longitudinal study of rhesus, they were more advanced in their disease and some were losing weight.

In addition, duration of obesity increased the risk for development of DM in humans [23]. There was a fourfold increase in the relative risk of the development of T2DM in human subjects who showed severe obesity (BMI ≥ 30) for less than 5 years. However, human subjects with severe obesity with duration lasting over 5 years showed an eightfold increase in relative risk of development of T2DM [22]. Therefore, baseline glucose level, degree of obesity, and duration of obesity increased risk for development of T2DM in humans. Interestingly, in this study, non-DM monkeys that continued to gain weight (peak body weight greater than their T2DM-matched peak body weight) over the 3 year beyond the study period eventually developed T2DM. In addition, all pre-DM monkeys were classified as obese; thus, as in humans, increased weight or excess adiposity is not a sufficient early predictor of overt T2DM.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Change in body weight/BF was not the precipitating factor in determining progression to overt T2DM. However, increased adiposity and body weight was present several years before overt DM in all rhesus monkeys that eventually progressed to DM.

Weight loss was shown to be a preceding diagnostic sign of progression of overweight primates toward T2DM. The maximum body weight occurred in most monkeys significantly before the onset of diabetes, most commonly 3-5 years before the overt disease.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References